The present invention relates to a method for measuring, by correlation, delays between two matching signals.
The measuring technique used is well known. It consists in sampling two electrical signals in two points respectively situated at a predetermined fixed distance from a movable medium having a non-homogenous type surface.
One of the signals is sampled at a first point and stored in a delay line, whereas the second signal is sampled at a second point and conveyed on a direct processing line. The method then consists in searching through the package of the delay line for the first signal which has a degree of resemblance with the second signal to enable a measurement of a delay by correlation. Such measurement permits, as a function of the distance between the two sampling points, to determine the speed of a moving object and/or the length covered between the two points through one time unit.
The measuring method used consists in calculating the correlation function between two signals sampled at the same frequency for the delay line and for the direct line, and in determining the delay corresponding to the maximum or peak of said function.
The principle of that technique is considered as well-known and accessible to anyone skilled in the art from numerous publications, so that it will not be necessary for the comprehension of the invention to describe it any further.
To carry out said function, the prior art technique suggests a correlation apparatus of a scientific nature permitting to obtain cross-correlation functions. But such apparatus may often by complex to use and do not give a direct reading of the delay linked to the time lag between two matching signals.
French Pat. No. 2 206 872 teaches that the correlation function can only be calculated from the polarity of the sampled signals. According to said French patent, it appears that the position of the cross-correlation peak, which is significant of the delay between two matching signals, gives an unbiased estimation of the delay, solely in the case of stationary signals.
There is another family of devices, called limited-delay devices or regulated sampling frequency devices. But said devices only calculate the cross-correlation function in its maximum point and they have the disadvantage of being regulated on parasitic peaks, non-significant of the cross-correlation function.
European Patent Application No. 0 026 877 describes a correlation apparatus for industrial use, applying the technique of coincidence of polarity. Such an apparatus comprises a microprocessor calculating only part of the approximated correlation function, and detecting the position of the peak in order to calculate and display the value of the delay to be measured. Said apparatus uses a controlled analysis window and a preprogrammed sampling frequency. Yet said apparatus has a relatively long measuring time, about one second, and so cannot be used for measuring delays particularly known to vary quickly and over a wide range.
When the range of delays to be measured is small, it is known to just calculate only a part of the cross-correlation function. This enables one to limit the number of points to be calculated during a sampling period, and as a result to limit the size of the memory. As a consequence, the computing power necessary to real-time working is reduced, as well as the risks of errors in peaks detection.
If with such means, the object is to process a wide range of delays, it is possible to move the window inside which the cross-correlation function is calculated, as this is taught in European Patent Application No. 0 026 877.
It is possible, in order to limit the size of the memory of the delay line, to provide a sub-sampling for one of the two lines as in European Application No. 0 026 877. But the disadvantage of this method is that it is slow.
It is also known from British Pat. No. 1 566 181, to use for the same purpose, a plurality of sampling frequencies, one of which is always in use in relation to the delay to be measured. The method has the disadvantage of creating an unusable transient state at each frequency change, since it is necessary to wait for the register of the delay line to be loaded with the samples taken at the new frequency before calculating the cross-correlation function.
The aforesaid methods enable to save on equipment and to quicken up the calculations. But these means are not altogether sufficient for measuring quick-varying delays over wide ranges, namely when the measures to be made concern signals sampled from a movable object undergoing sudden accelerations and decelerations.
The methods and apparatus of the aforedescribed type cannot therefore be used practically for measuring velocities and/or lengths travelled by industrial products manufactured in continuous manner, such as webs of paper or metal wires, or else, for assessing displacements in transports by rail or cable. In all these cases, the acceleration and decelerations can be considered as being high, so that the delivered signals are not of a sufficiently stationary nature to be processed by the known means.